I. Field of the Invention
This invention relates to a process for producing substantially unagglomerated single crystals of aluminum nitride and to the crystals so produced.
II. Description of the Prior Art
There is considerable interest nowadays in materials suitable for the reinforcement of metal, ceramic or polymeric matrix composites and similar materials. Composite materials of this type can be made to have very desirable properties, such as lightness of weight, high strength and resistance to high temperatures. Materials used for the reinforcement of the matrices must themselves have desirable properties, such as resistance to high temperatures and good resistance to attack by the matrix materials at high temperatures, as well as imparting a substantial reinforcing effect.
One material that is of interest as a matrix reinforcement is aluminum nitride. This material is especially suited for the reinforcement of aluminum matrix composites because of its high resistance to attack by molten aluminum. However, the reinforcing effect obtainable when using this material has been lower than expected because it has not been possible, at least on a commercial scale, to produce aluminum nitride in the form of particles ideally suited for reinforcement.
Aluminum nitride is used in the electronics industry, but the processes for the production of aluminum nitride for these applications generally result in the formation of sub-micron sized particles, which are too small for effective matrix reinforcement.
Most attempts to date to produce particles of aluminum nitride intended for reinforcement have resulted in polycrystalline particles which are then sintered and classified to obtain particles in the size range of at least about 10 microns. For example, individual particles can be produced by a carbothermal process for the production of aluminum nitride according to the following equation: EQU Al.sub.2 O.sub.3 +3C+N.sub.2 .fwdarw.2AlN+3CO
This reaction takes place at temperatures above 1700.degree. C. but, in order to grow crystals above 10 microns in diameter, the reaction temperature must be maintained above 1800.degree. C. However, when such temperatures are used, the product is unsatisfactory because a significant amount of carbon becomes trapped between agglomerating crystals of the AlN and the trapped carbon cannot be eliminated even by oxidation at elevated temperatures. Besides, a thick, almost sintered layer of AlN is formed on the surface of the particles, which hinders the diffusion of N.sub.2 inside the particle matrix as it is being formed, thus causing incomplete reaction in the interior of the particles.
Pending U.S. Pat. application Ser. No. 07/528,878 filed on May 25, 1990 in the name of S.K. Nadkarni and assigned to the same assignee as the present application, discloses a process for producing unagglomerated single crystals of AlN of at least 10 microns in size. The process involves reacting alumina with carbon under an atmosphere of nitrogen at a temperature in the range of 1800-1950.degree. C. in the presence of an alkali metal oxide as a crystal growth promoter or catalyst. The resulting particles are very useful for the reinforcement of composite materials, but the oxygen content of the particles can often be quite high and this reduces the thermal conductivity of the AlN, which is disadvantageous in many applications. The disclosure of this prior application is incorporated herein by reference.